This invention relates to a numerical control drive system and in particular to a numerical control drive system having a servo drive unit for driving a motor, a drive unit such as a spindle drive unit, a numerical control unit for outputting a control command for driving the motor to the two or more drive units, and a motor drive power converter for converting AC power into DC power and supplying the DC power to the two or more drive units.
FIG. 16 is a drawing to show the configuration of a numerical control (NC) drive system in a related art. In the Figure, numeral 41 denotes an AC power supply, numeral 42 denotes an AC reactor, numeral 43 denotes a motor drive power converter, numeral 44 denotes a spindle drive unit, numeral 45 denotes a spindle motor driven by the spindle drive unit 44, numeral 46 denotes a servo drive unit, numeral 47 denotes a servo motor driven by the servo drive unit 46, numeral 48 denotes an NC unit, and numeral 49 denotes a bus line. Numeral 50 denotes an AC-DC conversion circuit, numeral 51 denotes a diode, numeral 52 denotes a power module, and numeral 53 denotes a smoothing capacitor.
The motor drive power converter 43 rectifies AC power (L1, L2, L3) input via the AC reactor 42 from the AC power supply 41 to direct current through the diode 51, further smoothes the direct current through the smoothing capacitor 53 to produce DC power supply voltage VP, VN, to the spindle drive unit 44 and the servo drive unit 46. The spindle drive unit 44 and the servo drive unit 46 input the DC power supply voltage VP, VN and drive the spindle motor 45 and the servomotor 47 based on a position command from the NC unit 48.
When the servomotor 47 or the spindle motor 45 performs power running at the acceleration time, the AC-DC conversion circuit 50 rectifies alternating current to direct current and through the diode 51 supplies power. When the servomotor 47 or the spindle motor 45 runs in a regenerative mode at the deceleration time, the power module 52 switches so as to return the power to the AC power supply 41.
In recent years, with productivity improvement of a tool machine and development of technologies, shortening of the acceleration/deceleration time constant in a rapid feed mode and shortening of the cycle time have been demanded and further a larger torque (electric current) has been demanded at the acceleration/deceleration time and a servo drive unit and a spindle drive unit have been put into a high gain.
The demands for providing a high torque and a high gain described above have resulted in severe operating conditions of thermal stress of excessive current and power cycle in the motor drive power converter for supplying power to the servo drive unit and the spindle drive unit, and the capacity of the motor drive power converter has been increased as measures against heat of a diode and a power module and thus there are problems of upsizing and an increase in costs.
FIG. 17 is a block diagram of a variable-speed controller of an AC motor in a related art described in Japanese Patent Laid-Open No.85085/1986. In the Figure, numeral 61 denotes a motor for driving load of a fan, a pump, etc., numeral 62 denotes a position sensor being attached to the motor 61 for detecting the number of revolutions of the motor 61, and numeral 63 denotes an AC variable-speed drive for operating the motor 61 at variable speeds, an AC power supply being connected to the primary side of the AC variable-speed drive via a current transformer 64. Numeral 65 denotes is an overcurrent relay being connected to the secondary side of the current transformer 64 for stopping the operation of the AC variable-speed drive 63 to protect the motor if the motor is operated or accelerated in such a manner that the primary current value of the operation of the AC variable-speed drive 63 exceeds a rated value, and numeral 66 denotes a speed reference controller for setting speed reference. Numeral 67 denotes a current comparison controller for comparing the input current with a preset current limit value based on the input current supplied via the current transformer 64 to the AC variable-speed drive 63 and the number of revolutions of the motor input from the position sensor 62, and sends a control signal to the speed reference controller 66.
In a self-cooled motor with the cooling effect changing in response to the number of revolutions, the cooling effect changes in response to the number of revolutions and the current value limited from the heat resistance amount of the motor changes. A variable-speed controller of an AC motor in a related art is adapted to operate the motor within predetermined current limit values without stopping the motor by presetting the current limit value at the rated operation time and the current limit value at the acceleration time in all variable-speed area from the relationship between the cooling effect responsive to the number of revolutions and the heat resistance amount and when the input current reaches the current limitvalue at the related operation time in the rated operation mode, decreasing the number of revolutions of the motor for lowering the input current or when the input current reaches the current limit value at the related operation time in the acceleration operation mode, limiting an increase in the input current for temporarily stopping the acceleration.
In the variable-speed controller of an AC motor in the related art described above, if the input current of the AC variable-speed drive exceeds the rated value, the operation thereof is stopped by means of the overcurrent relay for protecting the motor or when the input current of the AC variable-speed drive is compared with the preset current limit value and the input current reaches the current limit value, the input current is lowered or an increase in the input current is limited, whereby protection against overheating is conducted without stopping the motor, namely, protection of the variable-speed controller is accomplished singly. In the whole NC drive system made up of the spindle drive unit 44 for driving the spindle motor 45, the servo drive unit 46 for driving the servomotor 47, etc., as shown in FIG. 16, if the spindle drive unit 44 or the servo drive unit 46 is equal to or less than the allowable current, whether or not an overcurrent occurs as the whole NC drive system cannot be determined; this is a problem.
The invention is intended for solving the problems as described above and it is an object of the invention to provide an NC drive system capable of performing stable motor control without increasing the capacity of a background motor drive power converter even in high-speed, high-acceleration drive.
According to the invention, there is provided a numerical control drive system having two or more drive units consisting of a servo drive unit for driving a motor, a spindle drive unit, etc., a numerical control unit for outputting a control command for driving the motor to the two or more drive units, and a motor drive power converter for converting AC power into DC power and supplying the DC power to the two or more drive units, wherein
the motor drive power converter comprises input current detection means for finding an input current, input current determination means for comparing the input current found by the input current detection means with an allowable current value with respect to less-than, equal-to, or greater-than relation, and control signal output means for outputting a control signal to the drive unit based on the determination result of the input current determination means, wherein the drive unit comprises control signal execution means for changing the control command from the numerical control unit based on the control signal output from the control signal output means, and wherein
if the input current determination means determines that the input current is greater than the allowable current value, the control signal execution means of the drive unit changes the control command from the numerical control unit, thereby lowering the input current.
If the input current determination means determines that the input current is greater than the allowable current value, the control signal execution means performs processing of lessening the inclination of a speed command.
If the input current determination means determines that the input current is greater than the allowable current value, the control signal execution means shuts off the gates of the switching elements of the drive units.
If the input current determination means determines that the input current is greater than the allowable current value, the control signal execution means clamps a speed command.
If the input current determination means determines that the input current is greater than the allowable current value, the control signal execution means clamps a motor drive current.
The motor drive power converter comprises phase detection means for detecting a power supply phase of the AC power and the input current determination means inputs the power supply phase detected by the phase detection means and compares the input current with the allowable current value with respect to the less-than, equal-to, or greater-than relation in the proximity of the power supply phase where the input current changes in direction.
The control signal output means inputs at least either motor drive currents or motor speeds from the two or more drive units to which the DC power is supplied, and selects the drive unit to which the control signal is to be output.
The motor drive power converter comprises cumulative-sum-of-times retention means for retaining the cumulative sum of times the input current determination means has determined that the input current exceeds the allowable current value, and alarm determination means for outputting an alarm to the drive units and the numerical control unit if the cumulative sum of times retained in the cumulative-sum-of-times retention means becomes equal to or greater than a reference value.
The motor drive power converter comprises input current output means for outputting the input current found by the input current detection means to the numerical control unit.